Optical router using interconnected photonic crystal elements with specific lattice-hole geometry

ABSTRACT

An optical routing element may include a planar dielectric photonic crystal which includes a lattice of holes having a first linear defect adjacent a second linear defect, with the two defects being separated by a central row of lattice holes. The first linear defect in the lattice of holes may form a first single mode line defect waveguide, and the second linear defect in the lattice of holes may form a second single mode line defect waveguide. Optical energy may be selectively coupled between the first and second waveguides across the central row of lattice holes. A free-carrier injector may be included to inject free-carriers into the dielectric photonic crystal, activation of which may alter selectivity of the optical coupling between the first and second waveguides. A plurality of optical routing elements with associated free-carrier injectors may be interconnected to form a bi-directional optical routing array.

PRIORITY

Priority is claimed as a continuation-in-part to U.S. patent applicationSer. No. 12/910,198, filed Oct. 22, 2010, now abandoned which claimspriority to U.S. provisional application Ser. No. 61/272,706, filed Oct.23, 2009. The disclosures of the aforementioned priority documents areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention may relate to optical routingelements and optical routing arrays.

2. Background

The International Technology Roadmap for Semiconductors highlights keyinterconnects challenges for next-generation microprocessors andcomputing systems. The roadmap suggests that the most difficultchallenges in the near term include the rapid introduction ofinterconnect processes compatible with device roadmaps, coupled withfine dimensional control and providing good mechanical stability andthermal budget. Further, the interconnect technologies should be able tomeet performance requirements and manufacturing targets by leveraginglow-cost conventional mass fabrication techniques and provide solutionsto address global wiring scaling issues. The continued push towardsfiner geometries, higher frequencies and larger chip sizes increasinglyexposes the disparity between interconnect needs and projectedinterconnect performance.

In order to realize reconfigurable computing, field programmable gatearrays (FPGAs) are vital, and hence there is a need to mimic neocorticinterconnect architectures, namely 3D routing with exceedingly highbandwidth density. In that regard, realization of 2D and 3D interconnectrouting topologies that use similar or compatible materials that achievebetter scale of integration and alignment tolerance would be extremelybeneficial.

Concepts for planar optical routing in a single layer of opto-electronicinterconnects using a planar self-collimation photonic crystal have beenproposed, as have a full three-dimensional interconnect using buriedsilicon micro-machining techniques. In the case of the slab, flip-chipsare bonded onto an underlying CMOS substrate that contains theappropriate driver and receiver circuitry to input and output opticalsignals to the slab. The slab also contains optical sources andreceivers that serve to generate and detect light. Within the slab, aself-collimation photonic crystal serves as the interconnect mediumbetween the source and detector. In the case of the buried siliconoptical interconnect technology, which may be referred to as thesub-surface silicon optical bus (S3B), direct integration into the CMOSprocess is easily achieved. The direction of propagation of the variousoptical signals as well as their destinations is achieved viaelectro-optical switches

Electro-optical switches are key components of such photonic integratedcircuits, yet only one proposal for implementing such switches—aresonator device—has appeared in the literature. The reconfigurablecomputing proposals may therefore benefit from additional options inelectro-optical switches.

SUMMARY OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention may be directed toward an opticalrouter, as a single optical routing element and/or as a plurality ofinterconnected optical routing elements (the latter is also referred toas a “routing fabric”). The optical routing element may be a photoniccrystal slow light based switch which utilizes electrically or opticallyinduced loss (conductivity). The photonic crystal may include twowaveguides between which optical energy is coupled.

In a first separate aspect of an embodiment of the present invention,the photonic crystal may be a planar dielectric photonic crystal, whichmay include a lattice of holes having a first linear defect adjacent asecond linear defect. The two linear defects may be separated by acentral row of lattice holes. The first linear defect may form a firstsingle mode line defect waveguide, and the second linear defect may forma second single mode line defect waveguide. Optical energy may beselectively coupled between the first and second waveguides across thecentral row of lattice holes. Optionally, the lattice of holes mayspatially taper adjacent the output coupling interfaces of eachrespective waveguide.

In a second separate aspect of an embodiment of the present invention,the optical routing element may include a free-carrier injectorconfigured to inject free-carriers into the photonic crystal. Activationof the free-carrier injector may alter optical coupling selectivitybetween the first and second waveguides. In one implementation, thefree-carrier injector, when activated, may be configured to alter therefractive index of the photonic crystal by at least 0.004.

In a third separate aspect of an embodiment of the present invention, aplurality of the optical routing elements may each include a photoniccrystal operatively coupled to a free-carrier injector. The photoniccrystals may be interconnected to form a bidirectional routing arrayhaving a plurality of input/output ports, such that selective activationof the photonic crystals with the respective free-carrier injectors mayenable routing of an optical signal from any one of the plurality ofports to any other of the plurality of ports.

In a fourth separate aspect of an embodiment of the present invention,the photonic crystal may be configured to couple optical energy at afirst wavelength between the waveguides while not coupling opticalenergy at a second wavelength between the waveguides. The first andsecond wavelengths may differ by about 0.4 nm to 0.8 nm.

In a fifth separate aspect of an embodiment of the present invention,the photonic crystal may be configured to couple optical energy betweenthe waveguides while exhibiting an extinction ratio of about −17 dB.

In a sixth separate aspect of an embodiment of the present invention,any of the foregoing aspects may be employed in combination.

According to an exemplary embodiment of the present invention, anoptical signal router may include a plurality of interconnected opticalrouting elements forming a bi-directional routing array having aplurality of optical signal ports, the plurality of ports including atleast four ports, each optical routing element including: a planardielectric photonic crystal including a lattice of holes having a firstlinear defect adjacent to a second linear defect, the first and secondlinear defects extending in a first direction and being separated by acentral row of lattice holes extending in the first direction whereinthe first linear defect in the lattice of holes forms a first singlemode line defect waveguide, and the second linear defect in the latticeof holes forms a second single mode line defect waveguide, such thatoptical energy of one or more electromagnetic waves propagating withinthe first and/or second waveguides is selectively coupled between thefirst and second waveguides in that such optical energy is enabled to bepartially or completely transferred between the first and secondwaveguides across the central row of lattice holes and a modulationdevice configured to modulate a concentration of free-carriers in thedielectric photonic crystal, thereby modulating optical couplingselectivity between the first and second waveguides across the centralrow of lattice holes, wherein the plurality of optical routing elementsare configured to enable routing an optical signal from any one of theplurality of ports to any other of the plurality of ports by selectiveactivation of the modulation device of one or more of the plurality ofoptical routing elements, and wherein the lattice holes for at least oneof the optical routing elements include a first, second, third, fourth,fifth and sixth row of lattice holes, each extending in the firstdirection, wherein the first, second and third rows of lattice holes areadjacent the first linear defect on a side of the first linear defectopposite to the location of the central row, with the second row oflattice holes located between the first and third row of lattice holes,and the first row of lattice holes located between the first lineardefect and the second row of lattice holes, wherein the fourth, fifthand sixth rows of lattice holes are adjacent the second linear defect ona side of the second linear defect opposite to the location of thecentral row, the fifth row of lattice holes located between the fourthand sixth row of lattice holes, the fourth row of holes located betweenthe second linear defect and the fourth row of lattice holes whereinconsecutive lattice holes of the second row of have a size larger than asize of consecutive lattice holes of the first row and consecutivelattice holes of the third row, and consecutive lattice holes of thefifth row have a size larger than a size of consecutive lattice holes ofthe fourth row and consecutive lattice holes of the sixth row.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, the free-carrier injectorincludes a PIN diode having an electrode disposed on each side of thelattice of holes of the respective dielectric photonic crystal.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, the free-carrier injectoris configured to alter a refractive index of the respective dielectricphotonic crystal by at least 0.004 when activated.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, optical energy at a firstwavelength is coupled between the waveguides, while optical energy at asecond wavelength, which differs from the first wavelength by about 0.4nm to 0.8 nm, is not coupled between the waveguides.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, optical energy selectivelycoupled between the waveguides exhibits an extinction ratio of about −17dB.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, each waveguide includes anoutput coupling interface, and the lattice of holes spatially tapersadjacent each output coupling interface.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, the lattice of holescomprises a periodic repetition of a transverse lattice element.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, a coupling region of thecentral row, in which optical energy coupling occurs, extends about 5transverse lattice elements.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, a row of only centralholes is disposed adjacent each waveguide, with a row seventh and eighthrows of only background lattice holes are respectively disposed betweenthe central row of lattice only central holes and each waveguide,wherein all the holes of the seventh and eighth rows of lattice holesare smaller than all the holes of the central row of lattice holes.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, the first waveguide isconfigured to propagate a first mode, and the second waveguide isconfigured to propagate a second mode different from the first mode.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, a width of the central rowis minimized by maximizing a separation of the first mode from thesecond mode in k-space.

According to an exemplary embodiment of the present invention, for theat least one of the optical routing elements, the first and secondwaveguides each include two coupling interfaces and are configured forbi-directional routing of optical signals.

According to an exemplary embodiment of the present invention, whereinthe at least one of the plurality of optical routing elements is notdirectly connected to one any of the plurality of ports without use ofanother of the plurality of optical routing elements.

According to an exemplary embodiment of the present invention, whereinthe lattice holes of the first, third, fourth and sixth row of holes allhave the same size.

According to an exemplary embodiment of the present invention, whereinthe lattice holes of the second, fifth and central row all have the samesize.

According to an exemplary embodiment of the present invention, whereinthe first row is immediately adjacent the first linear defect and thefourth row is immediately adjacent the second linear defect.

According to an exemplary embodiment of the present invention, whereinthe second row is immediately adjacent the first row and the fifth rowis immediately adjacent the fourth row.

According to an exemplary embodiment of the present invention, whereinthe consecutive lattice holes of the second row and consecutive latticeholes of the fifth row each comprise at least five consecutive latticeholes.

According to an exemplary embodiment of the present invention, whereinthe consecutive lattice holes of the first, second, third, fourth, fifthand sixth rows each comprise at least five consecutive lattice holes.

According to an exemplary embodiment of the present invention, whereinthe consecutive lattice holes of the second row are immediately adjacentto the consecutive lattice holes of the first row and immediatelyadjacent to the consecutive lattice holes of the third row, and theconsecutive lattice holes of the fifth row are immediately adjacent tothe consecutive lattice holes of the fourth row and immediately adjacentto the consecutive lattice holes of the sixth row.

According to an exemplary embodiment of the present invention, whereinthe consecutive lattice holes of the second row are immediately adjacentto the consecutive lattice holes of the first row and immediatelyadjacent to the consecutive lattice holes of the third row, and theconsecutive lattice holes of the fifth row are immediately adjacent tothe consecutive lattice holes of the fourth row and immediately adjacentto the consecutive lattice holes of the sixth row.

Accordingly, an improved optical router may be obtained. Advantages ofthe improvements will appear from the drawings and the description ofthe preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference numerals refer to similarcomponents:

FIG. 1 illustrates a bi-directional photonic crystal routing element;

FIGS. 2A & 2B show an image of a directional photonic crystal routingelement and a dispersion diagram associated therewith;

FIGS. 3A-3C show the simulated output from a directional photoniccrystal routing element;

FIG. 4 illustrates band diagrams corresponding to changes in the size ofholes between two waveguides;

FIG. 5 illustrates a comparison between calculated and experimentalresults from a directional photonic crystal routing element;

FIG. 6 shows the spectral response of a directional photonic crystalrouting element in a series of infrared images at various inputwavelengths;

FIG. 7 illustrates a band diagram showing the effect of changes in therefractive index in a directional photonic crystal routing element;

FIGS. 8A-8C show the simulated output from a switched directionalphotonic crystal routing element;

FIG. 9 illustrates changes in spectral output as compared to changes inthe index of refraction;

FIG. 10 illustrates the relationship between cross and bar states for aphotonic crystal routing element and moving between those two statesbased on changes in the phase mismatch;

FIG. 11 illustrates a directional photonic crystal routing element witha PIN diode;

FIGS. 12A-12D illustrate simulated results from a directional photoniccrystal routing element with a PIN diode;

FIGS. 13A-13C illustrate the design of a directional photonic crystalrouting element;

FIG. 14 illustrates the characteristics of a PIN diode when used withand without a directional photonic crystal routing element;

FIGS. 15A & 15B illustrate two different architectures for an opticalrouter incorporating a plurality of bi-directional photonic crystalrouting elements;

FIG. 16 illustrates an optical router and electrical and its opticalrouting performance;

FIG. 17 shows images of a bi-directional optical router;

FIGS. 18A and 18B illustrate experimental set-up and transmissionresults for a bi-directional optical router;

FIG. 19 shows an image of an eight port bi-directional optical router;

FIG. 20 illustrates a schematic diagram of the eight port bi-directionaloptical router shown in FIG. 19;

FIGS. 21A-21D illustrate four different routing options for an eightport bi-directional optical router;

FIG. 22 illustrates a reconfigurable optical cross-connect systemutilizing bi-directional optical routers;

FIG. 23 illustrates a parity based reconfigurable optical cross-connectsystem utilizing bi-directional optical routers;

FIG. 24 shows an image of an optical switch node which may beincorporated into an optical FPGA; and

FIG. 25 shows an image of a fabricated switch node with PIN diodes andelectrical contacts.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

The routing elements described herein may use photonic crystals (PhCs)along with the slow light effect in order to reduce and/or minimize thecoupling length needed to switch the optical beam between twowaveguides. By forming a defect in the PhC with a photonic band gap,photons can propagate only along the line defect, thus forming awaveguide. In the case of the coupler, two single mode waveguides may bebrought into close proximity to each other, forming a two mode system.These two modes, called even and odd modes, may propagate with differentgroup velocities, causing energy to flow from one waveguide into theother. The distance needed to achieve this coupling may generally berelated to the inverse of the separation between the wave numbers of thetwo modes. Therefore, maximizing the separation of the two modes ink-space may serve to minimize the coupling distance. This may beachieved using the slow light effect, which causes a sharp change in themode separation in k-space. Using this technique, coupling distances asshort as 5 μm may be obtained. The passive coupler, when combined withan active method for changing the refractive index, can be used as ahighly sensitive switch.

Turning in detail to the drawings, FIG. 1 illustrates a bi-directionalphotonic crystal routing element 11. The routing element 11 isillustrated as including two waveguides 13, 15 and four input/outputports 17 coupled to the waveguides 13, 15. The waveguides 13, 15 mayeach be formed by linear defects in a lattice of holes 19 formed on aplanar photonic crystal 21. The P and N electrodes 23, 25 of a PIN diodemay be disposed on opposite sides of the lattice 19. The PIN diode mayserve to change the refractive index of the PhC, thereby enabling use ofthe routing element 11 as an active optical switch.

The remainder of the disclosure below explores the properties of thisbidirectional photonic routing element (four input/output ports), alongwith those of the simpler directional photonic routing element (oneinput port and two output ports), and applications for each asinterconnected elements in routing fabric. Property and designhighlights for these elements and routers may include:

-   -   employing slow-light properties of planar photonic crystal        structures;    -   the slow-light properties (slow light factor=60) may enable        compact device design;    -   individual routing elements may be constructed with dimensions        of 10 μm×5 μm (˜20 photonic crystal lattice elements);    -   a routing node may have overall dimensions of 50 μm×50 μm;    -   full bi-directional routing may be enabled;    -   individual routing elements may be optimized to attain routing        for low-refractive index changes (Δn=0.004);    -   the power needed for routing may be optimized to less than 1 mW        per routing element;    -   the propagation loss may be about 0.0035 dB/μm, or 4×10⁻² dB per        routing element;    -   coupling between waveguides in a routing element may exhibit an        extinction ratio of about −17 dB;    -   the ability to filter and route C-band channels with channel        spacing of 4 nm and routing of C-band channels with channel        spacing of 0.4 nm;    -   pulse dispersion of 1.5 ps nm⁻¹ mm⁻¹ may be achieved;    -   eight channel routing of interconnected routing elements, using        various topologies, may be demonstrated;    -   active tuning elements may be obtained, along with optimized        device performance, for low routing power (1 mW) using optimal        doping concentrations (˜5×10¹⁸ cm⁻³);    -   the switching time for a routing element with active tuning        elements was experimentally demonstrated at 500 ns; and    -   a dispersion-free regime of up to 160 Gb/s may be obtained.        Design of a Slow-Light Based Nanophotonic Routing Element

As a solution to the growing demand for optical intra-chip communicationand routing, a dielectric 2D photonic crystal (PhC) directional coupler,which may be actively switched in plane, may be provided. This PhCdevice may be used to selectively couple light between two waveguides asa routing element, and it may be constructed having a device footprintless than 10 μm×10 μm. The PhC device that meets these designspecifications, as described below, may be referred to as a CoupledPhotonic Crystal Waveguide (CPhCWG). When compared to traditionaldielectric PhC couplers, the design of the CPhCWG may allow a largercoupling bandwidth with lower power consumption for active switching dueto the ability to have small device dimensions.

An image of a directional CPhCWG 31 is shown in FIG. 2A. Thisdirectional CPhCWG 31 image is of a fabricated directional CPhCWG in a260 nm Silicon-on-insulator (SOI) substrate. Similar devices could befabricated in a SOI thicknesses ranging from about 210 nm to 270 nm. Thedirectional CPhCWG 31 may include two coupled line defect photoniccrystal waveguides 33 forming a single input port 35 and two outputports, referred to herein as the bar port 37 and the cross port 39. Eachline defect waveguide may be formed by “removing” a row of holes in thePhC (i.e., not forming the row of holes during the manufacturingprocess, which is well-known to those of skill in the relevant arts),thereby restricting the propagation of light to only be in the directionof the removed row. The two proximal waveguides may then form a systemof modes with overlapping profiles, as shown in the dispersion diagramof FIG. 2B. Here, the two modes are referred to as being even 41 and odd43. The two modes may generally propagate in the waveguides withdifferent group velocities, resulting in exchange of optical powerbetween the two waveguides.

The directional CPhCWG 31 may include holes of two different sizes. Thesmaller holes 45 may form the bulk of the lattice, and the larger holes,referred to herein as central holes 47, may help control thecharacteristics of the modes that propagate in the waveguides. As isdiscussed in greater detail below, changing the diameter of the centralholes 47 may aid in fine-tuning characteristics such as couplingefficiency and coupling length. At times, it may be convenient tomeasure the coupling length in terms of transverse lattice elements; onetransverse lattice element 49 is shown outlined in FIG. 2A. Withappropriate choices in hole diameters, certain spectra of optical energymay be coupled between the waveguides in as few as five transverselattice elements.

The CPhCWG may utilize the slow light effect to reduce the physicaldevice length while maintaining a longer effective interaction lengthbetween the waveguides. This effect may be realized by engineering thedilation of hole diameters in the PhC to flatten the band of the evenmode. As a result of the light traveling slower in the PhC, the lightmay be subject to more of the material and device features, allowing forthe coupling length to be less than 10 μm. The increased interaction mayalso allow for a subtle index change of Δn=−0.004 to switch the CPhCWG'soutput from one port to the next. The device characteristics may differfrom traditional PhC couplers by offering a wider coupling bandwidth andreduced power requirements, but drawbacks may arise in the fabricationof the device. Because the optical properties of the device may beaffected by the device design, perhaps more than with other devices, thetolerance of fabrication dimensions and effect of defects may be greatlyamplified. For example, a dilation of 2 nm in a hole diameter of 330 nmmay result in a ˜5 nm shift of the coupling wavelengths. The enhancedmaterial interaction and low index change requirements may allow theCPhCWG to selectively couple between the two ports through free-carrierinjection of an in-plane PIN diode. By using a PIN diode, free-carriersmay be injected laterally across the PhC lattice, which may change therefractive index of the silicon and may directly shift the spectrum thatis coupled between the waveguides. The routing element may naturallyrest in the “OFF” state with no applied bias, coupling optical energydirected into the input port 35 to the cross port 39, and turning therouting element “ON” under forward bias, the optical spectrum mayblue-shift, resulting in the optical energy being coupled to the barport 37.

Such behavior can be observed through the finite-difference time-domain(FDTD) simulation results shown in FIGS. 3A-3C. FIG. 3A shows thesimulated output of the directional CPhCWG for the bar and cross ports.FIG. 3B illustrates a simulated directional CPhCWG coupling inputoptical energy to the bar port, and FIG. 3C illustrates a simulateddirectional CPhCWG coupling input optical energy to the cross port. Asdepicted, FIG. 3B may represent the “ON” state for a directional CPhCWG,and FIG. 3C may represent the “OFF” state. In particular, at thewavelength indicated by the arrow pointing from FIG. 3B to FIG. 3A,optical energy of the bar port may couple and transfer 301 to the crossport and may then transfer back 303 to the bar port. However, at thewavelength indicated by the arrow pointing from FIG. 3C to FIG. 3A,optical energy from the bar port may couple and may substantiallycompletely transfer 305 to the cross port. This may thus provide anillustration of the aforementioned “selective coupling” in that thecoupling of optical energy between the bar and cross ports may beselective based on wavelength of the optical signal. As has beendiscussed above and will be discussed further below this selectivecoupling may be controlled, e.g., by adjusting conditions/propertiesrelating to the CPhCWG, and may be used for purposes of switchingoptical signals.

Design of Routing Element Using PhC Directional Coupler

The dispersion properties of the coupled waveguide modes can beextracted using the Plane Wave Expansion Method (PWEM) where theelectromagnetic wave equation is solved as an eigenvalue problem with aperiodic boundary condition and using a coupled waveguide structure as aunit cell for the analysis.

FIG. 4 is a dispersion diagram for various mode profiles, each based ona different hole diameter in the PhC. From the dispersion diagram, it isevident that at certain frequencies, i.e., between 0.256 a/λ and 0.268a/λ, the wavevectors between even and odd modes are quite different. Atthe maximum point, the difference may be larger than 0.01. From thecoupling length formulation (Eq. (3) below), a coupling length of around5 lattices may be obtained. Such a short coupling length can be used forultra compact optical routing applications.

The dispersion properties of the coupled waveguide structure can betuned by changing the air hole sizes in the central row as well as thecladding surrounding both waveguides.

The dispersion diagrams with different air hole sizes are illustrated inFIG. 4. As the central hole diameter, d, increases from 0.6 a to 0.825a, both even and odd modes shift to higher frequencies. As d increasesto 0.825 a, there are two important features for the even mode asobserved from the dispersion diagrams. First, the slopes of two modesare almost parallel within the frequency range from 0.255 to 0.267 a/λ.The large frequency range indicates large operational bandwidth. On theother hand, at frequency of 0.267 a/λ, the dispersion curvature becomesflat at k=0.4. As is known from the definition of group velocity, thegroup index will be very large in this case, and a significant slowlight propagation in the photonic crystal waveguide can occur. As aresult, the strong EM coupling between even and odd modes may lead to anefficient energy transition between waveguides, which may be used toreduce the coupling length. To this end, by means of the fine-tuning ofair hole sizes, a large group index, which typically ranges from 40 to400, may be achieved.

Fabrication and Characterization of Single Routing Elements

A prototype of a routing element, as shown in FIG. 2, may be fabricatedusing e-beam lithography and an Inductively Couple Plasma Reactive IonEtch (ICP-RIE) with fluorine based chemistry for pattern transfer intothe SOI wafer. Experimental characterization of a sample fabricated inthis way was performed by fiber coupling an optical signal, supplied bya tunable laser source, into the photonic device. The routing elementstructure shown in FIG. 5 was simulated numerically with a couplingregion of 12.8 μm, and the routing element was experimentally fabricatedand spectrally characterized with the tunable lasers.

In FIG. 5, the lighter regions within the image of the two wavelengthspropagating through the device is scattered light from the waveguidesbeing supported by the underlying oxide. It should be noticed that thereis not a traceable amount of light detected that is scattering from thephotonic crystal routing element. The characterization of the singlephotonic crystal routing element presented the routing of twowavelengths with a separation of 1.16 nm across two ports with anaverage extinction ratio of −17 dB, agreeing with a numerical analysisof the routing element.

Spectral responses of fabricated single routing elements with varyingcoupling lengths were performed, demonstrating the tunable nature of thechannel spacing through the adjustment of the coupling length viainserting or removing the number of lattice sites that the photoniccrystal couples over. The separation of 6 nm between routablewavelengths, as shown in FIG. 6, where the input wavelength is tunedfrom 1550 nm to 1557 nm, illustrates the ability to passively routechannels within the International Telecommunication Union (ITU)standards for optical routing.

Tuning Routing Elements

By taking advantage of the slow light effect in the coupled PhCwaveguide, an active switching routing element can be designed using anonlinear medium. As an example, thermal-optic and electro-opticfree-carrier injection approaches may be applied to change therefractive index of the host material, thereby tuning the dispersionproperties. In such way, this compact coupler can be used to achievehighly sensitive and tunable optical devices, such as tunable opticalsplitter. One approach that can be used is to change the refractiveindex through free-carrier injection, where two electrodes arelithographically defined to apply a current through the PIN junction,with the PhC disposed within the junction. Through free-carrierinjection, the maximum change of index can be on the order of 0.01,which may provide enough dynamic range for the designed active routingelement.

To investigate the sensitivity of the optical routing element, therefractive index of the silicon host material was tuned down byΔn=−0.004 from the original index of 2.9. FIG. 7 illustrates thedispersion diagrams with refractive indices of 2.9 and 2.896. At afrequency of 0.2615 c/a, the wave vector difference of even and oddmodes at Δn=−0.004 are doubled compared with the case of Δn=0, whichmeans that the coupling length is reduced by a factor of two. This mayallow complete light switching to be achieved between the two waveguideswith an index change of as little as 0.004. Depending upon the materialsused, switching with yet smaller index changes may be achieved.

To confirm the design, a bi-directional routing element was simulated byusing the FDTD method. The design wavelength was chosen around 1500 nm.The background and central row air hole sizes were chosen as 240 nm and340 nm, respectively. Different hole sizes may be selected dependingupon the wavelength for which the routing element is designed. A totallength of 14 lattice elements was used for the coupling. The routingelement was fed with four dielectric waveguides. The dielectricwaveguide had a width of 690 nm. Due to the strong mismatch between thegroup velocity of the dielectric waveguide and that of the coupled PhCwaveguide, a spatial lattice tapering near the interface along thepropagation direction was introduced to minimize the unwanted interfacereflection. At both interfaces three PhC lattices were linearly taperedfrom 480 nm to 400 nm. Based on this design, an improved transmissionwas achieved in the simulations.

To characterize the spectrum response, a few detectors were placed atthe two output ports, labeled port 3 and port 4 in FIGS. 8B & 8C, torecord both transient and frequency spectra. FIG. 8A illustrates thesespectra, which show that at a wavelength of 1513.6 nm, ports 3 and 4achieve complete switching by an index change of 0.0004. FIGS. 8B & 8Cillustrates this complete switching by simulation of the power flowthrough in a routing element having a refractive index of 2.9 in FIG. 8Band a refractive index of 2.896 in FIG. 8C. The steady state results ofthe electrical fields in the computational region demonstrate the highthroughput coupling of the routing element.

In addition, the host material index may be continuously varied to studythe modulation at the wavelength of 1513.6 nm. The transmission responseof ports 3 and 4 versus the varying refractive index are illustrated inFIG. 9.

Actively Tuning Routing Elements

As has been indicated above, a routing element may be actively switched.For example, by applying an electric field, a change in the propagationconstant and hence the index of refraction in the coupling regionbetween the two waveguides, may be induced, and the system may changefrom a symmetric system of coupled PhC waveguides into a non-symmetricone. In this case the equation for the coupling length between the twowaveguides may be modified to include the change in the refractive indexcaused by the external applied field as follows:

$\begin{matrix}{L_{E} = \frac{\pi}{2\sqrt{( {\kappa^{2} + {\Delta^{2}n}} )}}} & (1)\end{matrix}$where L_(E) is the length for full power transfer under externalexcitation.

Mathematically, the operation of the routing element can be formulatedas being in either one of two states: the “bar” state (ON), when theenergy launched at the near end of one of the waveguides exits from thefar end of that same waveguide; and the “cross” state (OFF), when theenergy launched at the near end of one of the waveguides exits at thefar end of the other waveguide.

Using coupled mode theory, Equation (4) may be simplified to thefollowing(κL)²+(ΔβL)²=(νπ)².  (2)where ν is a positive integer, and where:

$\begin{matrix}{{{\kappa\; L} = {( {{2v} - 1} )\frac{\pi}{2}}},} & (3)\end{matrix}$where ν is again any positive integer. From Equation (3), the shortestlength for complete coupling is determined to occur when L=π/(2κ).

A plot of Equation (3) is shown in FIG. 10, which shows that the crossstates lie at isolated points on the axis Δβ=0 corresponding to thesynchronous case, whereas the bar states are represented by the arcs.Starting from a cross state, a bar state can be reached only by changingthe phase mismatch Δβ shown by the dotted line. Or mathematically,starting from a cross state with κL=π/2, then the required phasemismatch can be found from Equation (2) as

$\begin{matrix}{{\Delta\beta} = {{{\kappa\Delta}\; n} = {\sqrt{3}{\frac{\pi}{L}.}}}} & (4)\end{matrix}$

The phase mismatch condition can be achieved by applying an externalexcitation which may thus result in switching between the cross and thebar states.

The “loss tangent” of the dielectric material in the coupling region canbe modified by external “commands” to spoil the coupling, therebyre-routing the light. This may be characterized as an Δα switch (not theclassical Δβ switch), in which the change in optical absorptioncoefficient Δα is employed (the change in conductance Δσ is proportionalto Δα) as a modulating mechanism. The induced loss may not significantlyattenuate the waves traveling in the straight-through channels. Thisbehavior may be analogous to circumstances where electro-absorption hasbeen assumed to reduce the Q of micro-ring resonators coupled to stripchannel waveguides. To attain switching in the waveguides made fromSi/air or Si/SiO₂, the free-carrier absorption loss of Si can becontrolled by (1) free-carrier injection from forward-biased PNjunctions on the rods, (2) depletion of doped rods with MOS gates, or(3) generation of electrons and holes by above-gap light shining uponthe coupling area, a contact less process. If the routing element isimplemented in III-V semiconductor hetero-layers, then theelectro-absorption effect may be used. As is illustrated in FIG. 11, aPIN diode is positioned with the routing element in the intrinsic regionto introduce free-carriers into the PhC.

The design shown in FIG. 11 may need an extensive level of semiconductordevice modeling to determine the optimal doping concentrations anddoping profiles of the P and N regions, in order to minimize theelectrical power needed to route the optical signal between the variousoutput ports. Preferably, Phosphorous and Boron are used for the n-typeand p-type dopants, respectively, with concentrations for eachpreferably in the range of 10¹⁸ to 10¹⁹ per cm⁻³, and more preferablyhaving a concentration of about 5×10¹⁸ per cm⁻³. A commerciallyavailable package (Silvaco™) was used for detailed analysis of thedevice used in an implementation. FIGS. 12A-12D illustrate snapshots ofthe Silvaco™ analysis as well as the current and charge distributionthrough the device. FIG. 12A illustrates a cross-sectional view of arouting element showing the P and N doping regions and approximatelateral doping profile that is to be fabricated. The vertical dopingprofile is preferably approximately uniform throughout. FIG. 12Billustrates a lateral cross sectional view of a routing element showingthe intrinsic region. FIG. 12C illustrates the current densitydistribution in a routing element. FIG. 12D is a graph illustratingextracted I-V characteristics of simulated devices.

A directional routing element along with the PIN diode structure isshown in the design schematic of FIG. 13A. The gap between the P and Ndoping regions may be, e.g., 5 μm. The voltage across the PIN diode maybe controlled by the voltage source, which may enable the output of therouting element to be selected between the bar port and the cross port.FIG. 13B shows images of an exemplary routing element that wasconstructed with a PIN diode. In this image, the P-doped region is atthe top of the image, the N-doped region is at the bottom of the image,and the PhC is disposed between the two doped regions. FIG. 13C shows animage of silicon waveguides coupling in and out of an exemplary routingelement on a SOI platform, with gold contacts forming circuitconnections for the PIN diode. In FIG. 14, the performance of a PINdiode is compared with and without a PhC in the depletion region.

Design and Fabrication of Bi-Directional Optical Routers

Given the bi-directional routing element, e.g., as shown in FIG. 1several of these routing elements may be used as the basis for thedesign architecture of a bi-directional router. The architecture of twobasic bi-directional routers, both of which may be used as a routingnode for more complex routing fabrics, are shown in FIGS. 15A & 15B.Other types of routing nodes are possible, and the invention is not tobe limited to these embodiments. In either architecture, the opticalsignal between any adjacent or orthogonal ports can be redirected to anyother port externally by modulating the coupling between the twowaveguides of individual bi-directional routing elements. As such theoptical signal can travel in either the forward, backward, upward ordownward directions. This may provide an exceptional degree ofreconfigurability.

The routing performance, switching table, and propagation loss for anexample of a 2×2 routing fabric is shown in FIG. 16. In basic operation,an optical signal input at port A1 may pass through 4 elements to exitat port A2 and through 6 elements to exit at port B2. For the output atport A2, no routing elements may need to be ON, i.e., outputting opticalenergy at the bar port. For the output at port B2, only two routingelements, minimally, may need to be ON, which in an exemplaryembodiment, may consume approximately 1 mW of total electrical power toattain this path and may incur a cost of approximately 0.24 dB of totaloptical loss.

A prototype of a 1×3 bi-directional routing node was fabricated, as isshown photographically in FIG. 17. The node shown in photo (a) may allowan optical signal to be incident upon the routing fabric at any one ofthe four ports, and may route the signal to any other of the threeremaining ports. Photo (b) shows a single fundamental routing fabricelement of the routing fabric, which can be structured to an m×n array.Each fundamental routing fabric element may consist of four photoniccrystal elements joined by bending silicon nanowire waveguides. Photo(c) shows that the PhC routing element may utilize the input and outputof each line defect waveguide, which may allow each element to bebi-directional from every waveguide incident on the routing element. Thedensity of the routing elements within the fabric may be easily adjustedby changing the bending radius of the nanowire waveguides, either tospace out or condense the device footprint on a chip. In the fabricatedprototype, the approximate lateral and vertical dimension of the routingfabric element is 50 μm. Property and design highlights that may beachieved from such a routing fabric are listed in Table 1.

TABLE 1 Properties exhibited by an example routing fabric Routing unitcell 50 μm × 50 μm (node) dimensions Number of routing 4 elements perunit cell Propagation loss per 4 × 10⁻² dB/element routing elementSwitching (routing) 1 mW power per routing element Pulse dispersion 1.5ps nm⁻¹ mm⁻¹ Free dispersion rate Up to 160 Gb/s Routing speed 500 nsper element Extinction ratio −17 dB Routing channel 4 nm for routingelement of 16a coupling length spacing 2 nm for routing element of 36acoupling length 0.8 nm for routing element of 108a coupling length 0.4nm for routing element of 162a coupling length

To passively characterize a routing node, the exemplary routing nodepictured in FIG. 18A was constructed to allow monitoring of all possibleoutput channels represented by waveguides (WG) 1-6. Spectral scans wereperformed by launching an optical signal into the input nanowirewaveguide, and monitoring each output waveguide individually. Thespectral characterization showed the optical signal was appropriatelyrouted to waveguides WG3 and WG6 for their corresponding channels,maintaining a comparable performance of the single routing element witha channel separation of 1.16 nm and an average extinction ratio of about−12 dB. The measured outputs of WG3 and WG6 are shown in FIG. 18B. Theremaining 4 probed waveguides remained a minimum of −17 dB below thechannels optical signal strength. With this demonstration of acceptablesingle-node performance, the 8 port routing fabric shown in the photo ofFIG. 19 was fabricated using 4 routing nodes. As is explained below, the8 port routing fabric also performed as expected, and it is anticipatedthat routing fabrics with greater numbers of nodes, whether of similararchitecture or having significantly different architecture, but usingthe same basic routing element as the primary building block, could beconstructed using these same principles.

The network configurations and topologies presented below can beimplemented for the cases of a single incoming fiber carrying Ndifferent wavelength channels, or an N fiber ribbon carrying a number ofN different wavelength channels.

For an N port router, there are

$\prod\limits_{i = 0}^{({\frac{N}{2} - 1})}\;( {N - 1 - {2i}} )$ways to interconnect N-ports.

Therefore for N=8 there are 7×5×3=105 ways to connect an 8 port routingfabric. A conceptual design of an 8 port routing fabric, with port andelement labeling, is shown in FIG. 20. FIGS. 21A-21D illustrate 4possibilities out of the 112 ways to interconnect the 8 port routingfabric shown in FIG. 20. Each of these four different configurations maysimultaneously interconnect 4 pairs of the available 8 ports. In eachconfiguration, the routing elements in the ON state are listed in thecolumn adjacent the listed pair of ports. With these select routingelements in the ON state, a pathway between the two indicated ports maybe created for each configuration.

An optical crossconnect system in a reconfigurable optical network isillustrated in FIG. 22, which is an example of an optical crossconnectsystem utilizing a number of 16 port routing fabrics. Each routingfabric may interconnect an input from one of four channels within theoptical network to any one of another four channels within the opticalnetwork.

An example of a parity-based optical crossconnect system in areconfigurable optical network is illustrated in FIG. 23. Thisalternative to the system shown in FIG. 22 divides the incoming channelsinto odd and even channels and routes each channel based on its parity.

The routing element can further be implemented into a larger design as aswitch in an optical routing fabric. For example, the switch node, shownin the photo of FIG. 24 alongside a switching table, has thefunctionality to route an optical signal as a 1×3 directional switch.The switch node may be symmetric, allowing it to be replicated in a n×marray to form a large switch fabric that can function as an optical FPGAand perform operations such as routing, logic, integration andbuffering. Although, with the freedom to choose the size of the array,it does not necessarily mean that if more ports in the array are needed,a 2D chip layout can accommodate the extra real estate required. Asdevice densities continue to increase on chips, the ability to bereleased from the restrictions of photonic devices structured in a 2Dsubstrate can allow for a new 3D photonic chip architecture by layeringdevices and building vertically on the chip. An alternative fabricatedswitch node is shown in the photos of FIG. 25, which also illustratesthe PIN diode electrical contacts.

Thus, an optical signal router, both as a single optical signal routingelement and as a plurality of interconnected optical signal routingelements, may be provided. While embodiments of this invention have beenshown and described, it will be apparent to those skilled in the artthat many more modifications are possible without departing from theinventive concepts herein. The invention, therefore, is not to berestricted except in the spirit of the following claims.

What is claimed is:
 1. An optical signal router comprising: a pluralityof interconnected optical routing elements forming a bi-directionalrouting array having a plurality of optical signal ports, the pluralityof ports including at least four ports, each optical routing elementcomprising: a planar dielectric photonic crystal including a lattice ofholes having a first linear defect adjacent to a second linear defect,the first and second linear defects extending in a first direction andbeing separated by a central row of lattice holes extending in the firstdirection, wherein the first linear defect in the lattice of holes formsa first single mode line defect waveguide, and the second linear defectin the lattice of holes forms a second single mode line defectwaveguide, such that optical energy of one or more electromagnetic wavespropagating within the first and/or second waveguides is selectivelycoupled between the first and second waveguides in that such opticalenergy is enabled to be partially or completely transferred between thefirst and second waveguides across the central row of lattice holes; anda modulation device configured to modulate a concentration offree-carriers in the dielectric photonic crystal, thereby modulatingoptical coupling selectivity between the first and second waveguidesacross the central row of lattice holes, wherein the plurality ofoptical routing elements are configured to enable routing an opticalsignal from any one of the plurality of ports to any other of theplurality of ports by selective activation of the modulation device ofone or more of the plurality of optical routing elements, and whereinthe lattice holes for at least one of the optical routing elementsinclude a first, second, third, fourth, fifth and sixth row of latticeholes, each extending in the first direction, wherein the first, secondand third rows of lattice holes are adjacent the first linear defect ona side of the first linear defect opposite to the location of thecentral row, with the second row of lattice holes located between thefirst and third row of lattice holes, and the first row of lattice holeslocated between the first linear defect and the second row of latticeholes, wherein the fourth, fifth and sixth rows of lattice holes areadjacent the second linear defect on a side of the second linear defectopposite to the location of the central row, the fifth row of latticeholes located between the fourth and sixth row of lattice holes, thefourth row of holes located between the second linear defect and thefourth row of lattice holes wherein consecutive lattice holes of thesecond row of have a size larger than a size of consecutive latticeholes of the first row and consecutive lattice holes of the third row,and consecutive lattice holes of the fifth row have a size larger than asize of consecutive lattice holes of the fourth row and consecutivelattice holes of the sixth row.
 2. The optical signal router of claim 1,wherein for the at least one of the optical routing elements, thefree-carrier injector comprises a PIN diode having an electrode disposedon each side of the lattice of holes of the respective dielectricphotonic crystal.
 3. The optical signal router of claim 1, wherein forthe at least one of the optical routing elements, the free-carrierinjector is configured to alter a refractive index of the respectivedielectric photonic crystal by at least 0.004 when activated.
 4. Theoptical signal router of claim 1, wherein for the at least one of theoptical routing elements, optical energy at a first wavelength iscoupled between the waveguides, while optical energy at a secondwavelength, which differs from the first wavelength by about 0.4 nm to0.8 nm, is not coupled between the waveguides.
 5. The optical signalrouter of claim 1, wherein for the at least one of the optical routingelements, optical energy selectively coupled between the waveguidesexhibits an extinction ratio of about −17 dB.
 6. The optical signalrouter of claim 1, wherein for the at least one of the optical routingelements, each waveguide includes an output coupling interface, and thelattice of holes spatially tapers adjacent each output couplinginterface.
 7. The optical signal router of claim 1, wherein for the atleast one of the optical routing elements, the lattice of holescomprises a periodic repetition of a transverse lattice element.
 8. Theoptical signal router of claim 1, wherein for the at least one of theoptical routing elements, a coupling region of the central row, in whichoptical energy coupling occurs, extends about 5 transverse latticeelements.
 9. The optical signal router of claim 1, wherein for the atleast one of the optical routing elements, a row of only central holesis disposed adjacent each waveguide, with a row seventh and eighth rowsof only background lattice holes are respectively disposed between thecentral row of lattice only central holes and each waveguide, whereinall the holes of the seventh and eighth rows of lattice holes aresmaller than all the holes of the central row of lattice holes.
 10. Theoptical signal router of claim 1, wherein for the at least one of theoptical routing elements, the first waveguide is configured to propagatea first mode, and the second waveguide is configured to propagate asecond mode different from the first mode.
 11. The optical signal routerof claim 10, wherein for the at least one of the optical routingelements, a width of the central row is minimized by maximizing aseparation of the first mode from the second mode in k-space.
 12. Theoptical signal router of claim 1, wherein for the at least one of theoptical routing elements, the first and second waveguides each includetwo coupling interfaces and are configured for bi-directional routing ofoptical signals.
 13. The optical signal router of claim 1, wherein theat least one of the plurality of optical routing elements is notdirectly connected to one any of the plurality of ports without use ofanother of the plurality of optical routing elements.
 14. The opticalsignal router of claim 1, wherein the lattice holes of the first, third,fourth and sixth row of holes all have the same size.
 15. The opticalsignal router of claim 1, wherein the lattice holes of the second, fifthand central row all have the same size.
 16. The optical signal router ofclaim 1, wherein the first row is immediately adjacent the first lineardefect and the fourth row is immediately adjacent the second lineardefect.
 17. The optical signal router of claim 16, wherein the secondrow is immediately adjacent the first row and the fifth row isimmediately adjacent the fourth row.
 18. The optical signal router ofclaim 1, wherein the consecutive lattice holes of the second row andconsecutive lattice holes of the fifth row each comprise at least fiveconsecutive lattice holes.
 19. The optical signal router of claim 1,wherein the consecutive lattice holes of the first, second, third,fourth, fifth and sixth rows each comprise at least five consecutivelattice holes.
 20. The optical signal router of claim 19, wherein theconsecutive lattice holes of the second row are immediately adjacent tothe consecutive lattice holes of the first row and immediately adjacentto the consecutive lattice holes of the third row, and the consecutivelattice holes of the fifth row are immediately adjacent to theconsecutive lattice holes of the fourth row and immediately adjacent tothe consecutive lattice holes of the sixth row.
 21. The optical signalrouter of claim 1, wherein the consecutive lattice holes of the secondrow are immediately adjacent to the consecutive lattice holes of thefirst row and immediately adjacent to the consecutive lattice holes ofthe third row, and the consecutive lattice holes of the fifth row areimmediately adjacent to the consecutive lattice holes of the fourth rowand immediately adjacent to the consecutive lattice holes of the sixthrow.